Electrical interview question

Electrical interview questions By

JAYASEELAN CJ

2015

2/7/2015

www.jaicj.in [email protected]

Electrical interview questions 2015

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Questions from transformer 1. Which of the following does not change in a

transformer?

(a) Current

(b) Voltage

(c) Frequency

(d) All of the above

Ans: c

2. In a transformer the energy is conveyed from

primary to secondary

(a) through cooling coil

(b) through air

(c) by the flux

(d) none of the above

Ans: c

3. A transformer core is laminated to

(a) reduce hysteresis loss

(b) reduce eddy current losses

(c) reduce copper losses

(d) reduce all above losses

Ans: b

4. The degree of mechanical vibrations produced by

the laminations of a transformer depends on

(a) tightness of clamping

(b) gauge of laminations

(c) size of laminations

(d) all of the above

Ans: d

5. The no-load current drawn by transformer is

usually what per cent of the full-load current ?

(a) 0.2 to 0.5 per cent

(b) 2 to 5 per cent

(c) 12 to 15 per cent

(d) 20 to 30 per cent

Ans: b

6. The path of a magnetic flux in a transformer

should have

(a) high resistance

(b) high reluctance

(c) low resistance

(d) low reluctance

Ans: d

7. No-load on a transformer is carried out to

determine

(a) copper loss

(b) magnetising current

(c) magnetising current and loss

(d) efficiency of the transformer

Ans: c

8. The dielectric strength of transformer oil is

expected to be

(a) lkV

(b) 33 kV

(c) 100 kV

(d) 330 kV

Ans: b

9. Sumpner's test is conducted on trans-formers to

determine

(a) temperature

(b) stray losses

(c) all-day efficiency


Ans: a

10. The permissible flux density in case of cold

rolled grain oriented steel is around

(a) 1.7 Wb/m2

(b) 2.7 Wb/m2

(c) 3.7 Wb/m2

(d) 4.7 Wb/m2

Ans: a

11. The efficiency of a transformer will be

maximum when

(a) copper losses = hysteresis losses

(b) hysteresis losses = eddy current losses

(c) eddy current losses = copper losses

(d) copper losses = iron losses

Ans: d

12. No-load current in a transformer

(a) lags behind the voltage by about 75°

(b) leads the voltage by about 75°

(c) lags behind the voltage by about 15°

(d) leads the voltage by about 15°

Ans: a

13. The purpose of providing an iron core in a

transformer is to

(a) provide support to windings(b) reduce

hysteresis loss

(c) decrease the reluctance of the magnetic path

(d) reduce eddy current losses

Ans: c

14. Which of the following is not a part of

transformer installation ?

(a) Conservator

(b) Breather

(c) Buchholz relay

(d) Exciter

Ans: d

15. While conducting short-circuit test on a

transformer the following side is short circuited

(a) High voltage side

(b) Low voltage side

(c) Primary side

(d) Secondary side

Ans: b

16. In the transformer following winding has got

more cross-sectional area

(a) Low voltage winding

(b) High voltage winding

(c) Primary winding

(d) Secondary winding

Ans: a

17. A transformer transforms



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(a) voltage

(b) current

(c) power

(d) frequency

Ans: c

18. A transformer cannot raise or lower the voltage

of a D.C. supply because

(a) there is no need to change the D.C. voltage

(b) a D.C. circuit has more losses

(c) Faraday's laws of electromagnetic induction are

not valid since the rate of change of flux is zero


Ans: c

19. Primary winding of a transformer

(a) is always a low voltage winding

(b) is always a high voltage winding

(c) could either be a low voltage or high voltage

winding


Ans: c

20. Which winding in a transformer has more

number of turns ?

(a) Low voltage winding

(b) High voltage winding

(c) Primary winding


Ans: b

21. Efficiency of a power transformer is of the

order of

(a) 100 per cent

(b) 98 per cent

(c) 50 per cent

(d) 25 per cent

Ans: b

22. In a given transformer for given applied

voltage, losses which remain constant irrespective

of load changes are

(a) friction and windage losses

(b) copper losses

(c) hysteresis and eddy current losses


Ans: c

23. A common method of cooling a power

transformer is

(a) natural air cooling

(b) air blast cooling

(c) oil cooling

(d) any of the above

Ans: c

24. The no load current in a transformer lags behind

the applied voltage by an angle of about

(a) 180°

(b) 120"

(c) 90°

(d) 75°

Ans: d

25. In a transformer routine efficiency depends

upon

(a) supply frequency

(b) load current

(c) power factor of load

(d) both (b) and (c)

Ans: d

26. In the transformer the function of a conservator

is to

(a) provide fresh air for cooling the transformer

(b) supply cooling oil to transformer in time of need

(c) protect the transformer from damage when oil

expends due to heating


Ans: c

27. Natural oil cooling is used for transformers up

to a rating of

(a) 3000 kVA

(b) 1000 kVA

(c) 500 kVA

(d) 250 kVA

Ans: a

28. Power transformers are designed to have

maximum efficiency at

(a) nearly full load

(b) 70% full load

(c) 50% full load

(d) no load

Ans: a

29. The maximum efficiency of a distribution

transformer is

(a) at no load

(b) at 50% full load

(c) at 80% full load

(d) at full load

Ans: b

30. Transformer breaths in when

(a) load on it increases

(b) load on it decreases

(c) load remains constant


Ans: b

31. No-load current of a transformer has

(a) has high magnitude and low power factor

(b) has high magnitude and high power factor

(c) has small magnitude and high power factor

(d) has small magnitude and low power factor

Ans: d

32. Spacers are provided between adjacent coils

(a) to provide free passage to the cooling oil

(b) to insulate the coils from each other

(c) both (a) and (b)


Ans: a

33. Greater the secondary leakage flux

(a) less will be the secondary induced e.m.f.



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(b) less will be the primary induced e.m.f.

(c) less will be the primary terminal voltage


Ans: a

34. The purpose of providing iron core in a step-up

transformer is

(a) to provide coupling between primary and

secondary

(b) to increase the magnitude of mutual flux

(c) to decrease the magnitude of mag-netizing

current

(d) to provide all above features

Ans: c

35. The power transformer is a constant

(a) voltage device

(b) current device

(c) power device

(d) main flux device

Ans: d

36. Two transformers operating in parallel will

share the load depending upon their

(a) leakage reactance

(b) per unit impedance

(c) efficiencies

(d) ratings

Ans: b

37. If R2 is the resistance of secondary winding of

the transformer and K is the transformation ratio

then the equivalent

secondary resistance referred to primary will be

(a) R2/VK

(b) R2IK2

(c) R22!K2

(d) R22/K

Ans: b

38. What will happen if the transformers working in

parallel are not connected with regard to polarity ?

(a) The power factor of the two trans-formers will

be different from the power factor of common load

(b) Incorrect polarity will result in dead short

circuit

(c) The transformers will not share load in

proportion to their kVA ratings


Ans: b

39. If the percentage impedances of the two

transformers working in parallel are different, then

(a) transformers will be overheated

(b) power factors of both the transformers will be

same

(c) parallel operation will be not possible

(d) parallel operation will still be possible, but the

power factors at which the two transformers operate

will be different

from the power factor of the common load

Ans: d

40. In a transformer the tappings are generally

provided on

(a) primary side

(b) secondary side

(c) low voltage side

(d) high voltage side

Ans: c

41. The use of higher flux density in the

transformer design

(a) reduces weight per kVA

(6) reduces iron losses

(c) reduces copper losses

(d) increases part load efficiency

Ans: a

42. The chemical used in breather for transformer

should have the quality of

(a) ionizing air

(b) absorbing moisture

(c) cleansing the transformer oil

(d) cooling the transformer oil.

Ans: b

43. The chemical used in breather is

(a) asbestos fiber

(b) silica sand

(c) sodium chloride

(d) silica gel

Ans: d

44. An ideal transformer has infinite values of

primary and secondary inductances. The statement

is

(a) true

(b) false

Ans: b

45. The transformer ratings are usually expressed in

terms of

(a) volts

(b) amperes

(c) kW

(d) kVA

Ans: d

46. The noise resulting from vibrations of

laminations set by magnetic forces, is termed as

(a) magnetostrication

(b) boo

(c) hum

(d) zoom

Ans: c

47. Hysteresis loss in a transformer varies as

CBmax = maximum flux density)

(a) Bmax

(b) Bmax1-6

(C) Bmax1-83

(d) B max

Ans: b

48. Material used for construction of transformer

core is usually



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(a) wood

(b) copper

(c) aluminium

(d) silicon steel

Ans: d

49. The thickness of laminations used in a

transformer is usually

(a) 0.4 mm to 0.5 mm

(b) 4 mm to 5 mm

(c) 14 mm to 15 mm

(d) 25 mm to 40 mm

Ans: a

50. The function of conservator in a transformer is

(a) to project against'internal fault

(b) to reduce copper as well as core losses

(c) to cool the transformer oil

(d) to take care of the expansion and contraction of

transformer oil due to variation of temperature of

sur-roundings

Ans: d

51. The highest voltage for transmitting electrical

power in India is

(a) 33 kV.

(6) 66 kV

(c) 132 kV

(d) 400 kV

Ans: d

52. In a transformer the resistance between its

primary and secondary is

(a) zero

(b) 1 ohm

(c) 1000 ohms

(d) infinite

Ans: d

53. A transformer oil must be free from

(a) sludge

(b) odour

(c) gases

(d) moisture

Ans: d

54. A Buchholz relay can be installed on

(a) auto-transformers

(b) air-cooled transformers

(c) welding transformers

(d) oil cooled transformers

Ans: d

55. Gas is usually not liberated due to dissociation

of transformer oil unless the oil temperature

exceeds

(a) 50°C

(b) 80°C

(c) 100°C

(d) 150°C

Ans: d

56. The main reason for generation of harmonics in

a transformer could be

(a) fluctuating load

(b) poor insulation

(c) mechanical vibrations

(d) saturation of core

Ans: d

57. Distribution transformers are generally

designed for maximum efficiency around

(a) 90% load

(b) zero load

(c) 25% load

(d) 50% load

Ans: d

58. Which of the following property is not

necessarily desirable in the material for transformer

core ?

(a) Mechanical strength

(6) Low hysteresis loss

(c) High thermal conductivity

(d) High permeability

Ans: c

59. Star/star transformers work satisfactorily when

(a) load is unbalanced only

(b) load is balanced only

(c) on balanced as well as unbalanced loads


Ans: b

60. Delta/star transformer works satisfactorily when

(a) load is balanced only

(b) load is unbalanced only

(c) on balanced as well as unbalanced loads


Ans: c

61. Buchholz's relay gives warning and protection

against

(a) electrical fault inside the transformer itself

(b) electrical fault outside the transformer in

outgoing feeder

(c) for both outside and inside faults


Ans: a

62. The magnetising current of a transformer is

usually small because it has

(a) small air gap

(b) large leakage flux

(c) laminated silicon steel core

(d) fewer rotating parts

Ans: a

63. Which of the following does not change in an

ordinary transformer ?

(a) Frequency

(b) Voltage

(c) Current

(d) Any of the above

Ans: a



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64. Which of the following properties is not

necessarily desirable for the material for

transformer core ?

(a) Low hysteresis loss

(b) High permeability

(c) High thermal conductivity

(d) Adequate mechanical strength

Ans: c

65. The leakage flux in a transformer depends upon

(a) load current

(b) load current and voltage

(c) load current, voltage and frequency

(d) load current, voltage, frequency and power

factor

Ans: a

66. The path of the magnetic flux in transformer

should have

(a) high reluctance

(b) low reactance

(c) high resistance

(d) low resistance

Ans: b

67. Noise level test in a transformer is a

(a) special test

(b) routine test

(c) type test


Ans: c

68. Which of the following is not a routine test on

transformers ?

(a) Core insulation voltage test

(b) Impedance test

(c) Radio interference test

(d) Polarity test

Ans: c

69. A transformer can have zero voltage regulation

at

(a) leading power factor

(b) lagging power factor

(c) unity power factor

(d) zero power factor

Ans: a

70. Helical coils can be used on

(a) low voltage side of high kVA transformers

(b) high frequency transformers

(c) high voltage side of small capacity transformers

(d) high voltage side of high kVA rating

transformers

Ans: a

71. Harmonics in transformer result in

(a) increased core losses

(b) increased I2R losses

(c) magnetic interference with communication

circuits


Ans: d

72. The core used in high frequency transformer is

usually

(a) copper core

(b) cost iron core

(c) air core

(d) mild steel core

Ans: c

73. The full-load copper loss of a trans¬former is

1600 W. At half-load, the copper loss will be

(a) 6400 W

(b) 1600 W

(c) 800 W

(d) 400 W

Ans: d

1.74. The value of flux involved m the e.m.f.

equation of a transformer is

(a) average value

(b) r.m.s. value

(c) maximum value

(d) instantaneous value

Ans: c

L.75. Silicon steel used in laminations mainly

reduces

(a) hysteresis loss

(b) eddy current losses

(c) copper losses


Ans: a

76. Which winding of the transformer has less

cross-sectional area ?

(a) Primary winding

(b) Secondary winding

(c) Low voltage winding

(d) High voltage winding

Ans: d

77. Power transformers are generally designed to

have maximum efficiency around

(a) no-load

(b) half-load

(c) near full-load

(d) 10% overload

Ans: c

.78. Which of the following is the main advantage

of an auto-transformer over a two winding

transformer ?

(a) Hysteresis losses are reduced

(b) Saving in winding material

(c) Copper losses are negligible

(d) Eddy losses are totally eliminated

Ans: b

79. During short circuit test iron losses are

negligible because

(a) the current on secondary side is negligible

(b) the voltage on secondary side does not vary

(c) the voltage applied on primary side is low



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(d) full-load current is not supplied to the

transformer

Ans: c

80. Two transformers are connected in parallel.

These transformers do not have equal percentage

impedance. This is likely to

result in

(a) short-circuiting of the secondaries

(b) power factor of one of the transformers is

leading while that of the other lagging

(c) transformers having higher copper losses will

have negligible core losses

(d) loading of the transformers not in proportion to

their kVA ratings

Ans: d

81. The changes in volume of transformer cooling

oil due to variation of atmospheric temperature

during day and

night is taken care of by which part of transformer

(a) Conservator

(b) Breather

(c) Bushings

(d) Buchholz relay

Ans: a

82. An ideal transformer is one which has

(a) no losses and magnetic leakage

(b) interleaved primary and secondary windings

(c) a common core for its primary and secondary

windings

(d) core of stainless steel and winding of pure

copper metal

(e) none of the above

Ans: a

83. When a given transformer is run at its rated

voltage but reduced frequency, its

(a) flux density remains unaffected

(b) iron losses are reduced

(c) core flux density is reduced

(d) core flux density is increased

Ans: d

84. In an actual transformer the iron loss remains

practically constant from noload to fullload because

(a) value of transformation ratio remains constant

(b) permeability of transformer core remains

constant

(c) core flux remains practically constant

(d) primary voltage remains constant

(c) secondary voltage remains constant

Ans: c

85. An ideal transformer will have maximum

efficiency at a load such that

(a) copper loss = iron loss

(b) copper loss < iron loss

(c) copper loss > iron loss


Ans: a

86. If the supply frequency to the transformer is

increased,"the iron loss will

(a) not change

(b) decrease

(c) increase


Ans: c

87. Negative voltage regulation is indicative that

the load is

(a) capacitive only

(b) inductive only

(c) inductive or resistive


Ans: a

88. Iron loss of a transformer can be measured by

(a) low power factor wattmeter

(b) unity power factor wattmeter

(c) frequency meter

(d) any type of wattmeter

Ans: a

89. When secondary of a current transformer is

open-circuited its iron core will be

(a) hot because of heavy iron losses taking place in

it due to high flux density

(b) hot because primary will carry heavy current

(c) cool as there is no secondary current

(d) none of above will happen

Ans: a

90. The transformer laminations are insulated from

each other by

(a) mica strip

(6) thin coat of varnish

(c) paper


Ans: b

91. Which type of winding is used in 3phase shell-

type transformer ?

(a) Circular type

(b) Sandwich type

(c) Cylindrical type

(d) Rectangular type

Ans: b

92. During open circuit test of a transformer

(a) primary is supplied rated voltage

(b) primary is supplied full-load current

(c) primary is supplied current at reduced voltage

(d) primary is supplied rated kVA

Ans: a

93. Open circuit test on transformers is conducted

to determine

(a) hysteresis losses

(b) copper losses

(c) core losses

(d) eddy current losses

Ans: c



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94. Short circuit test on transformers is conducted

to determine

(a) hysteresis losses

(b) copper losses

(c) core losses

(d) eddy current losses

Ans: b

95. For the parallel operation of single phase

transformers it is necessary that they should have

(a) same efficiency

(b) same polarity

(c) same kVA rating

(d) same number of turns on the secondary side.

Ans: b

96. The transformer oil should have _____

volatility and _____ viscosity.

(a) low,low

(b) high,high

(c) low,high

(d) high,low

Ans: a

97. The function of breather in a transformer is

(a) to provide oxygen inside the tank

(b) to cool the coils during reduced load

(c) to cool the transformer oil

(d) to arrest flow of moisture when outside air

enters the transformer

Ans: d

98. The secondary winding of which of the

following transformers is always kept closed ?

(a) Step-up transformer

(b) Step-down transformer

(c) Potential transformer

(d) Current transformer

Ans: d

99. The size of a transformer core will depend on

(a) frequency

(b) area of the core

(c) flux density of the core material

(d) (a) and (b) both

Ans: d

100. N atural air coo ling is generally restricted for

transformers up to

(a) 1.5 MVA

(b) 5 MVA

(c) 15 MVA

(d) 50 MVA

Ans: a

101. A shell-type transformer has

(a) high eddy current losses

(b) reduced magnetic leakage

(c) negligibly hysteresis losses


Ans: b

102. A transformer can have regulation closer to

zero

(a) on full-load

(b) on overload

(c) on leading power factor

(d) on zero power factor

Ans: c

103. A transformer transforms

(a) voltage

(b) current

(c) current and voltage

(d) power

Ans: d

104. Which of the following is not the standard

voltage for power supply in India ?

(a) llkV

(b) 33kV

(c) 66 kV

(d) 122 kV

Ans: d

105. Reduction in core losses and increase in

permeability are obtained with transformer

employing

(a) core built-up of laminations of cold rolled grain

oriented steel

(b) core built-up of laminations of hot rolled sheet

(c) either of the above


Ans: a

106. In a power or distribution transformer about 10

per cent end turns are heavily insulated

(a) to withstand the high voltage drop due to line

surge produced by the shunting capacitance of the

end turns

(b) to absorb the line surge voltage and save the

winding of transformer from damage

(c) to reflect the line surge and save the winding of

a transformer from damage


Ans: a

107. For given applied voltage, with the increase in

frequency of the applied voltage

(a) eddy current loss will decrease

(b) eddy current loss will increase

(c) eddy current loss will remain unchanged


Ans: c

108. Losses which occur in rotating electric

machines and do not occur in trans formers are

(a) friction and windage losses

(b) magnetic losses

(c) hysteresis and eddy current losses

(d) copper losses

Ans: a

109. In a given transformer for a given applied

voltage, losses which remain constant irrespective

of load changes are

(a) hysteresis and eddy current losses



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(b) friction and windage losses

(c) copper losses


Ans: a

110. Which of the following statements regarding

an idel single-phase transformer having a turn ratio

of 1 : 2 and

drawing a current of 10 A from 200 V A.C. supply

is incorrect ?

(a) Its secondary current is 5 A

(b) Its secondary voltage is 400 V

(c) Its rating is 2 kVA

(d) Its secondary current is 20 A

(e) It is a step-up transformer

Ans: d

111. The secondary of a current transformer is

always short-circuited under operating conditions

because it

(a) avoids core saturation and high voltage

induction

(b) is safe to human beings

(c) protects the primary circuit


Ans: a

112. In a transformer the resistance between its

primary and secondary should be

(a) zero

(b) 10 Q

(c) 1000 Q

(d) infinity

Ans: d

113. A good voltage regulation of a transformer

means

(a) output voltage fluctuation from no load to full

load is least

(b) output voltage fluctuation with power factor is

least

(c) difference between primary and secondary

voltage is least

(d) difference between primary and secondary

voltage is maximum

Ans: a

114. For a transformer, operating at constant load

current, maximum efficiency will occur at

(a) 0.8 leading power factor

(b) 0.8 lagging power factor

(c) zero power factor

(d) unity power factor

Ans: d

115. Which of the following protection is normally

not provided on small distribution transformers ?

(a) Overfluxing protection

(b) Buchholz relay

(c) Overcurrent protection

(d) All of the above

Ans: b

116. Which of the following acts as a protection

against high voltage surges due to lightning and

switching ?

(a) Horn gaps

(b) Thermal overload relays

(c) Breather

(d) Conservator

Ans: a

117. The efficiency of two identical transformers

under load conditions can be determined by

(a) short-circuit test

(b) back-to-back test

(c) open circuit test


Ans: b

118. Which of the following insulating materials

can withstand the highest temperature safely ?

(a) Cellulose

(b) Asbestos

(c) Mica

(d) Glass fibre

Ans: c

119. Which of the following parts of a transformer

is visible from outside ?

(a) Bushings

(b) Core

(c) Primary winding


Ans: a

120. The noise produced by a transformer is termed

as

(a) zoom

(b) hum

(c) ringing

(d) buzz

Ans: b

121. Which of the following loss in a transformer is

zero even at full load ?

(a) Core loss

(b) Friction loss

(c) Eddy current loss

(d) Hysteresis loss

Ans: b

122. Which of the following is the most likely

source of harmonics in a transformer ?

(a) poor insulation

(b) Overload

(c) loose connections

(d) Core saturation

Ans: d

123. If a transformer is continuously operated the

maximum temperature rise will occur in

(a) core

(b) windings

(c) tank




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Ans: b

124. The hum in a transformer is mainly attributed

to

(a) load changes

(b) oil in the transformer

(c) magnetostriction

(d) mechanical vibrations

Ans: c

125. The maximum load that a power transformer

can carry is limited by its

(a) temperature rise

(b) dielectric strength of oil

(c) voltage ratio

(d) copper loss

Ans: c

126. The efficiency of a transformer, under heavy

loads, is comparatively low because

(a) copper loss becomes high in proportion to the

output

(b) iron loss is increased considerably

(c) voltage drop both in primary and secondary

becomes large

(d) secondary output is much less as compared to

primary input

Ans: a

127. An open-circuit test on a transformer is

conducted primarily to measure

(a) insulation resistance

(b) copper loss

(c) core loss

(d) total loss

(e) efficiency

(f) none of the above

Ans: c

128. A no-load test is performed on a transformer

to determine

(a) core loss

(b) copper loss

(c) efficiency

(d) magnetising current

(e) magnetising current and loss

Ans: e

129. The voltage transformation ratio of a

transformer is equal to the ratio of

(a) primary turns to secondary turns

(b) secondary current to primary current

(c) secondary induced e.m.f. to primary induced

e.m.f.

(d) secondary terminal voltage to primary applied

voltage

Ans: c

130. Part of the transformer which is most subject

to damage from overheating is

(a) iron core

(b) copper winding

(c) winding insulation

(d) frame or case

(e) transformer tank

Ans: c

131. If a transformer is switched on to a voltage

more than the rated voltage

(a) its power factor will deteriorate

(b) its power factor will increase

(c) its power factor will remain unaffected

(d) its power factor will be zero

Ans: a

132. Auto-transformer makes effective saving on

copper and copper losses, when its transformation

ratio is

(a) approximately equal to one

(b) less than one

(c) great than one


Ans: a

133. Minimum voltage regulation occurs when the

power factor of the load is

(a) unity

(b) lagging

(c) leading

(d) zero

Ans: c

134. In a step-down transformer, there is a change

of 15 A in the load current. This results in change

of supply current of

(a) less than 15 A

(b) more than 15 A

(c) 15 A


Ans: a

135. The efficiencies of transformers compared

with that of electric motors of the same power are

(a) about the same

(6) much smaller

General questions

1: What is electric traction?

Answer: Traction means using the electric power

for traction system i.e. for railways, trams, trolleys

etc. electric traction means use of the electricity for

all these. Now a day, magnetic traction is also used

for bullet trains. Basically dc motors are used for

electric traction systems.

2: How can you start-up the 40w tube light with

230v AC/DC without using any choke/Coil?



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Answer: It's possible by means of Electronic

choke. otherwise it's not possible to ionize the

particles in tube. light, with normal voltage.

3: What is "pu" in electrical engineering?

Answer: Pu stands for per unit and this will be

used in power system single line diagram there it is

like a huge electrical circuit with no of components

(generators, transformers, loads) with different

ratings (in MVA and KV). To bring all the ratings

into common platform we use pu concept in which,

in general largest MVA and KV ratings of the

component is considered as base values, then all

other component ratings will get back into this

basis. Those values are called as pu values.

(p.u=actual value/base value).

4: Operation carried out in Thermal power station?

Answer: The water is obtained in the boiler and the

coal is burnt so that steam is obtained this steam is

allowed to hit the turbine , the turbine which is

coupled with the generator generates the electricity

5:Why link is provided in neutral of an ac circuit

and fuse in phase of ac circuit?

Answer: Link is provided at a Neutral common

point in the circuit from which various connection

are taken for the individual control circuit and so it

is given in a link form to withstand high Amps. But

in the case of Fuse in the Phase of AC circuit it is

designed such that the fuse rating is calculated for

the particular circuit (i.e load) only.So if any

malfunction happen the fuse connected in the

particular control circuit alone will blow off.

6: what is the diff. btwn. Electronic regulator and

ordinary rheostat regulator for fans?

Answer: The difference between the electronic and

ordinary regulator is that in electronic reg. power

losses are less because as we decrease the speed the

electronic reg. give the power needed for that

particular speed but in case of ordinary rheostat

type reg. the power wastage is same for every speed

and no power is saved. In electronic regulator triac

is employed for speed control. By varying the firing

angle speed is controlled but in rheostat control

resistance is decreased by steps to achieve speed

control.

7: How tube light circuit is connected and how it

works?

Answer: A choke is connected in one end of the

tube light and a starter is in series with the circuit.

When supply is provided , the starter will interrupt

the supply cycle of AC. Due to the sudden change

of supply the chock will generate around 1000volts

. This volt will capable of to break the electrons

inside the tube to make electron flow. once the

current passes through the tube the starter circuit

will be out of part. now there is no change of

supply causes choke voltage normalized and act as

minimize the current.

8: whats is MARX CIRCUIT?

Answer: It is used with generators for charging a

number of capacitor in parallel and discharging

them in series.It is used when voltage required for

testing is higher than the available.

9:What is encoder, how it function?

Answer: An encoder is a device used to change a

signal (such as a bit stream) or data into a code. The

code may serve any of a number of purposes such

as compressing information for transmission or

storage, encrypting or adding redundancies to the

input code, or translating from one code to another.

This is usually done by means of a programmed

algorithm,especially if any part is digital, while

most analog encoding is done with analog circuitry.

10:What are the advantages of speed control

using thyristor?

Answer: Advantages :1. Fast Switching

Characteristics than MOSFET, BJT, IGBT 2. Low

cost 3. Higher Accurate.

11: Why Human body feel Electric shock ?? n in an

Electric train during running , We did nt feel any

Shock ? why?

Answer: Unfortunately our body is a pretty good

conductor of electricity, The golden rule is Current

takes the lowest resistant path if you have

insulation to our feet as the circuit is not complete

(wearing rubber footwear which doing some repairs

is advisable as our footwear is a high resistance

path not much current flows through our body).The

electric train is well insulated from its electrical

system.



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12 :what is the principle of motor?

Answer: Whenever a current carrying conductor is

placed in a magnetic field it produce turning or

twisting movement is called as torque.

13: why, when birds sit on transmission lines or

current wires doesn't get shock?

Answer: It’s true that if birds touch the single one

line (phase or neutral) they don't get electrical

shock... if birds touch 2 lines than the circuit is

closed and they get electrical shock.. so if a human

touch single one line(phase) then he doesn't get

shock if he is in the air (not touching - standing on

the ground if he is standing on the ground then

touching the line (phase) he will get a shock

because the ground on what we standing is like line

(ground bed - like neutral)। and in the most of

electric lines the neutral is grounded..so that means

that human who touch the line closes the circuit

between phase and neutral.

14:what is meant by armature reaction?

Answer: The effect of armature flu to main flux is

called armature reaction. The armature flux may

support main flux or opposes main flux.

15: what happen if we give 220 volts dc supply to d

bulb r tube light?

Answer: Bulbs [devices] for AC are designed to

operate such that it offers high impedance to AC

supply. Normally they have low resistance. When

DC supply is applied, due to low resistance, the

current through lamp would be so high that it may

damage the bulb element.

16: Which motor has high Starting Torque and

Staring current DC motor, Induction motor or

Synchronous motor?

Answer:DC Series motor has high starting torque.

We can not start the Induction motor and

Synchronous motors on load, but can not start the

DC series motor without load.

17: what is ACSR cable and where we use it?

Answer: ACSR means Aluminium conductor steel

reinforced, this conductor is used in transmission &

distribution.

18: What is vaccum circuit breaker.define with

cause and where be use it Device?

Answer: A breaker is normally used to break a

circuit. while breaking the circuit, the contact

terminals will be separated. At the time of

separation an air gap is formed in between the

terminals. Due to existing current flow the air in the

gap is ionized and results in the arc. various

mediums are used to quench this arc in respective

CB's. but in VCB the medium is vacuum gas. since

the air in the CB is having vacuum pressure the arc

formation is interrupted. VCB's can be used up to

11kv.

20: What will happen when power factor is leading

in distribution of power?

Answer: If there is high power factor, i.e if the

power factor is close to one:

1.losses in form of heat will be reduced,

2.cable becomes less bulky and easy to carry, and

very

cheap to afford, &

3. it also reduces over heating of transformers.

21: what’s the one main difference between UPS &

inverter? And electrical engineering & electronics

engineering?

Answer: uninterrupted power supply is mainly use

for short time . means according to ups VA it gives

backup. Ups is also two types : on line and offline .

online ups having high volt and amp for long time

backup with high dc voltage. But ups start with 12v

dc with 7 amp. but inverter is start with 12v,24,dc

to 36v dc and 120amp to 180amp battery with long

time backup.

22: What is 2 phase motor?

Answer: A two phase motor is a motor with the

starting winding and the running winding have a

phase split. e.g;AC servo motor. Where the

auxiliary winding and the control winding have a

phase split of 90 degree.

23: Advantages of vvvf drives over non vvvf drives

for EOT cranes?



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Answer: 1.smooth start and stop.

2. No jerking of load.

3. Exact positioning

4. Better protection for motor.

5. high/low speed selection.

6. Reliability of break shoe.

7. Programmable break control.

8. Easy circuitry

9. Reduction in controls

10. Increases motor life

24:What is the significance of vector grouping in

Power Transformers?

Answer:Every power transformer has a vector

group listed by its manufacturer. Fundamentally it

tells you the information about how the windings

are connected (delta or wye) and the phace

difference betweent the current and voltage. EG.

DYN11 means Delta primary, Wye Secondry and

the current is at 11 o clock reffered to the voltage.

25:Which type of A.C motor is used in the fan

(ceiling fan, exhaust fan, padestal fan, bracket fan

etc) which are find in the houses ?

Answer:Its Single Phase induction motor which

mostly squirrel cage rotor and are capacitor start

capacitor run.

26:Give two basic speed control scheme of DC

shunt motor?

Answer: 1. By using flux control method: in this

method a rheostat is connected across the field

winding to control the field current. So by changing

the current the flux produced by the field winding

can be changed, and since speed is inversely

proportional to flux speed can be controlled

2.armature control method: in this method a

rheostat is connected across armature winding by

varying the resistance the value of resistive

drop(IaRa)can be varied, and since speed is directly

proportional to Eb-IaRa the speed can be

controlled.

27: what is the principle of motor?

Answer: Whenever a current carrying conductor is

placed in an magnetic field it produce turning or

twisting movement is called as torque

28: what is meant by armature reaction?

Answer: The effect of armature flu to main flux is

called armature reaction. The armature flux may

support main flux or opposes main flux.

29:

30:what is the difference between synchronous

generator & asynchronous generator?

Answer:In simple, synchronous generator supply's

both active and reactive power but asynchronous

generator(induction generator) supply's only active

power and observe reactive power for

magnetizing.This type of generators are used in

windmills

31:What is the Polarization index value ? (pi

value)and simple definition of polarization index ?

Answer: Its ratio between insulation

resistance(IR)i.e meggar valuefor 10min

to insulation resistance for 1 min. It ranges from 5-

7 for new motors & normally for motor to be in

good condition it should be Greater than 2.5 .

32: Why syn. generators r used for the production

of electricity?

Answer:synchronous machines have capability to

work on different power factor(or saydifferent

imaginary power varying the field emf. Hence syn.

generators r used for the production of electricity.

33: What is the difference between synchronous

generator & asynchronous generator?

Answer: In simple, synchronous generator supply's

both active and reactive power but asynchronous

generator (induction generator) supply's only active

power and observe reactive power for magnetizing.

This type of generators are used in windmills.



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34:1 ton is equal to how many watts?

Answer:1 ton = 12000 BTU/hr and to convert

BTU/hr to horsepower,

12,000 * 0.0003929 = 4.715 hp therefore 1 ton =

4.715*.746 = 3.5 KW.

35:why syn. generators r used for the production of

electricity?

Answer:synchronous machines have capability to

work on differentpower factor(or saydifferent

imaginary pow varying the field emf. Hence syn.

generators r used for the production of electricity.

36: Enlist types of dc generator?

Answer: D.C.Generators are classified into two

types 1)separatly exicted d.c.generator 2)self

exicted d.c.generator, which is further classified

into;1)series 2)shunt and3)compound(which is

further classified into cumulative and differential).

37:What’sAutomatic Voltage regulator(AVR)?

Answer: AVR is an abbreviation for

Automatic Voltage Regulator.It is important part in

Synchronous Generators, it controls

theoutput voltage of the generator by controlling its

excitation current. Thus it can control the output

Reactive Power of the Generator.

38: What is an exciter and how does it work?

Answer: There are two types of exciters, static

exciter and rotory exciter.purpose of excitor is to

supply the excitation dc voltage to the fixed poles

of generator.Rotory excitor is an additional small

generator mounted on the shaft of main generator.

if it is dc generator, it will supply dc to the rotory

poles through slip ring and brushes( conventional

alternator). if it is an ac excitor, out put of ac

excitor is rectified by rotating diodes and supply dc

to main fixed poles.ac excitor is the ac generator

whose field winding are stationary and armature

rotates. initialvoltage is built up by residual

magnetism.It gives the starting torque to the

generator.

40: Difference between a four point starter and

three point starter?

Answer: The shunt connection in four point stater

is provided separately form the line where as in

three point stater it is connected with line which is

the drawback in three point stater

41: Why use the VCB at High Transmission

System ? Why can't use ACB?

Answer: Actually the thing is vacuum has high arc

queching property compare to air because in VCB

,the die electric strengths equal to 8 times of air .

That y always vaccum used as inHT breaker and air

used as in LT .

42: What is the difference between surge arrestor

and lightning arrestor?

Answer: LA is installed outside and the effect of

lightning is grounded,where as surge arrestor

installed inside panels comprising of resistors

which consumes the energy and nullify the effect of

surge.

43: What happens if i connect a capacitor to a

generator load?

Answer:Connecting a capacitor across a generator

always improves powerfactor,but it will help

depends up on the engine capacity of the

alternator,other wise the alternator will be over

loaded due to the extra watts consumed due to

the improvement on pf. Secondly, don't connect a

capacitor across an alternator while it is picking up

or without any other load.

44: Why the capacitors works on ac only?

Answer: Generally capacitor gives infinite

resistance to dc components(i.e., block the dc

components). it allows the ac components to pass

through.

45: Explain the working principal of the circuit

breaker?

Answer: Circuit Breaker is one which makes or

breaks the circuit.It has two contacts namely fixed

contact & moving contact.under normal condition

the moving contact comes in contact with fixed

contact thereby forming the closed contact for the

flow of current. During abnormal & faulty

conditions(when current exceeds the rated value) an

arc is produced between the fixed & moving

contacts & thereby it forms the open circuitArc is



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extinguished by the Arc Quenching media like air,

oil, vaccum etc.

46: How many types of colling system it

transformers?

Answer: 1. ONAN (oil natural,air natural)2. ONAF

(oil natural,air forced)3. OFAF (oil forced,air

forced)4. ODWF (oil direct,water forced) 5. OFAN

(oil forced,air forced)

47: What is the function of anti-pumping in circuit

breaker?

Answer: when breaker is close at one time by close

push button,the anti pumping contactor prevent re

close the breaker by close push button after if it

already close.

48: what is stepper motor.what is its uses?

Answer: Stepper motor is the electrical machine

which act upon input pulse applied to it. it is one

type of synchronous motor which runs in steps in

either direction instead of running in complete

cycle.so, in automation parts it is used.

49: How to calculate capacitor bank value to

maintain unity power factor with some suitable

example?

Answer:KVAR= KW(TAN(COS(-1)#e)-

TAN(COS(-1)#d) )

#e= EXISTING P.F.

#d= DESIRED P.F.

50: Tell me in detail about c.t. and p.t.

?(Company:reliance)

Answer:The term C.T means current

transformer,and the term P.T means potential

transformer.In circuit where measurements of high

voltage and high current is involved they are used

there.Particularly when a measuring device like

voltmeter or ammeter is not able to measure such

high value of quantity because of large value of

torque due to such high value it can damage the

measuring device.so, CT and PT are introduced in

the circuits. They work on the same principle of

transformer, which is based on linkage of

electromagneticflux produced by primary with

secondary.They work on the ratio to they are

designed.E.g if CTis of ratio 5000\5A and it has to

measure secondary current of

8000A.then ANS=8000*5\5000=8Aand this result

will be given to ammeter .and after measuring 8A

we can calculate the primary current.same is

the operation of PT but measuring voltage.

51: There are a Transformer and an induction

machine. Those two have the same supply. For

which device the load current will be maximum?

And why?

Answer: The motor has max load current compare

to that of transformer because the motor consumes

real power.. and the transformer is only producing

the working flux and its not consuming.. hence the

load current in the transformer is because of core

loss so it is minimum.

52: what is power factor? whether it should be high

or low? why?

Answer:Power factor should be high in order to get

smooth operation of the system.Low power factor

means losses will be more.it is the ratio of true

power to apperent power. it has to be ideally 1. if it

is too low then cable over heating & equipment

overloading will occur. if it is greater than 1 then

load will act as capacitor and starts feeding the

source and will cause tripping.(if pf is poor ex: 0.17

to meet actual power load has to draw more

current(V constant),result in more lossesif pf is

good ex: 0.95 to meet actual power load has to

draw less current(V constant),result in less losses).

53:What is the difference between Isolator

and Circuit Breaker?

Answer:Isolator is a off load device which is used

for isolating the downstream circuits from upstream

circuits for the reason of any maintenance on

downstream circuits. it is manually operated and

does not contain any solenoid unlike circuit

breaker. it should not be operated while it is having

load. first the load on it must be made zero and then

it can safely operated. its specification only rated

current is given.But circuit breaker is onload

automatic device used for breaking the circuit in

case of abnormal conditions like short circuit,



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overload etc., it is having three specification 1 is

rated current and 2 is short circuit breaking capacity

and 3 is instantaneous tripping current.

54:what is boucholz relay and the significance of it

in to the transformer?

Answer:Boucholz relay is a device which is used

for the protection of transformer from its internal

faults, it is a gas based relay. whenever any internal

fault occurs in a transformer, the boucholz relay at

once gives a horn for some time, if the transformer

is isolated from the circuit then it stop its sound

itself other wise it trips the circuit by its own

tripping mechanism.

55:What is SF6 Circuit Breaker?

Answer:SF6 is Sulpher hexa Flouride gas.. if this

gas is used as arc quenching medium in a

Circuitbreaker means SF6 CB.

56:what is ferrantic effect?

Answer:Output voltage is greater than the input

voltage or receiving end voltage is greater than the

sending end voltage.

The Ferranti effect is a rise in voltage occurring at

the receiving end of a long transmission line,

relative to the voltage at the sending end, which

occurs when the line is energized but there is a very

light load or the load is disconnected.

This effect is due to the voltage drop across the line

inductance (due to charging current) being in phase

with the sending end voltages. Therefore both

capacitance and inductance are responsible for

producing this phenomenon.[1]

The Ferranti Effect will be more pronounced the

longer the line and the higher the voltage

applied.[2] The relative voltage rise is proportional

to the square of the line length.[3]

Due to high capacitance, the Ferranti effect is much

more pronounced in underground cables, even in

short lengths.

57: what is meant by insulation voltage in cables?

explain it?

Answer:It is the property of a cable by virtue of it

can withstand the applied voltage without rupturing

it is known as insulation level of the cable.

58: Why we do 2 types of earthing on transformer

(ie:)body earthing & neutral earthing , what is

function. i am going to install a 5oo kva

transformer & 380 kva DG set what should the

earthing value?

Answer:The two types of earthing are Familiar as

Equipment earthing and system earthing. In

Equipment earthing: body ( non conducting part)of

the equipment shouldd be earthed to safegaurd the

human beings.system Earthing : In this neutral of

the supply source ( Transformer or Generator)

should be grounded. With this,in case of

unbalanced loading neutral will not be shifted.so

that unbalanced voltages will not arise. We can

protect the equipment also. With size of the

equipment( transformer or alternator)and selection

of relying system earthing will be further classified

into directly earthed,Impedance earthing, resistive

(NGRs) earthing.

59:What is the difference between MCB & MCCB,

Where it can be used?

Answer:MCB is miniature circuit breaker which is

thermal operated and use for short circuit protection

in small current rating circuit. MCCB moulded case

circuit breaker and is thermal operated for over load

current and magnetic operation for instant trip in

short circuit condition.under voltage and under

frequency may be inbuilt. Normally it is used where

normal current is more than 100A.

60:Where should the lighting arrestor be placed in

distribution lines?

Answer:Near distribution transformers and out

going feeders of 11kv and incomming feeder of

33kv and near power transformers in sub-stations.

61:Define IDMT relay?

Answer:It is an inverse definite minimum time

relay.In IDMT relay its operating is inversely

proportional and also a characteristic of minimum

time after which this relay operates.It is inverse in

the sense ,the tripping time will decrease as the

magnitude of fault current increase.



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62:What are the transformer losses?

Answer:TRANSFORMER LOSSES - Transformer

losses have two sources-copper loss and magnetic

loss. Copper losses are caused by the resistance of

the wire (I2R). Magnetic losses are caused by eddy

currents and hysteresis in the core. Copper loss is a

constant after the coil has been wound and

therefore a measurable loss. Hysteresis loss is

constant for a particular voltage and current. Eddy-

current loss, however, is different for each

frequency passed through the transformer.

63:What is meant by regenerative braking?

Answer:When the supply is cutt off for a running

motor, it still continue running due to inertia. In

order to stop it quickly we place a load(resitor)

across the armature winding and the motor should

have maintained continuous field supply. so that

back e.m.f voltage is made to applyacross the

resistor and due to load the motor stops

quickly.This type of breaking is called as

"Regenerative Breaking".

64:Why is the starting current high in a DC motor?

Answer:In DC motors, Voltage equation is V=Eb-

IaRa (V = Terminal voltage,Eb = Back emf in

Motor,Ia = Armature current,Ra = Aramture

resistance).At starting, Eb is zero. Therefore,

V=IaRa, Ia = V/Ra ,where Ra is very less like

0.01ohm.i.e, Ia will become enormously increased.

65:What are the advantages of star-delta starter

with induction motor?

Answer:(1). The main advantage of using the star

delta starter is reduction of current during the

starting of the motor.Starting current is reduced to

3-4 times Of current of Direct online starting.(2).

Hence the starting current is reduced , the voltage

drops during the starting of motor in systems are

reduced.

66:Why Delta Star Transformers are used for

Lighting Loads?

Answer:For lighting loads, neutral conductor is

must and hence the secondary must be star winding.

and this lighting load is always unbalanced in all

three phases. To minimize the current unbalance in

the primary we use delta winding in the primary. So

delta / star transformer is used for lighting loads.

67:Why in a three pin plug the earth pin is thicker

and longer than the other pins?

Answer:It depends upon R=rho l/a where area(a) is

inversely proportional to resistance (R), so if (a)

increases, R decreases & if R is less the leakage

current will take low resistance path so the earth pin

should be thicker. It is longer because the The First

to make the connection and Last to disconnnect

should be earth Pin. This assures Safety for the

person who uses the electrical instrument.

Electrical interview question:Why series motor

cannot be started on no-load?

Answer:Series motor cannot be started without load

because of high starting torque. Series motor are

used in Trains, Crane etc.

68:Why ELCB can't work if N input of ELCB do

not connect to ground?

Answer:ELCB is used to detect earth leakage fault.

Once the phase and neutral are connected in an

ELCB, the current will flow through phase and that

much current will have to return neutral so resultant

current is zero. Once there is a ground fault in the

load side, current from phase will directly pass

through earth and it will not return through neutral

through ELCB. That means once side current is

going and not returning and hence because of

this difference in current ELCB wil trip and it will

safe guard the other circuits from faulty loads. If

the neutral is not grounded, fault current will

definitely high and that full fault current will come

back through ELCB, and there will be

no difference in current.

69:How electrical power is generated by an A.C

Generator?

Answer:For the generation of elect power we need

a prime mover which supplies mechanical power

input to the alternator, can be steam turbines,or

hydro turbines .When poles of the rotor moves

under the armature conductors which are placed on

the stator ,field flux cut the

armature conductor ,therefore voltage is generated

and is of sinusoidal in nature...due to polarity

change of rotor poles(i,e) N-S-N-S.



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70:Why an ac solenoid valve attract the plunger

even though we interchanges the terminal? Will the

poles changes?

Answer:Yes because the poles changes for every

half-cycle of ac voltageso the polarity of

AC voltage is continuously changing for every half

cycle. so, interchanging of terminals in ac system

does not show any difference. That's why the ac

solenoid attract the plunger even though it's

terminals are interchanged.

71:What is derating?, why it is necessary, it is same

for all means for drives, motors,and cables.

Answer:The current currying of cables will change

depending upon the site temperature (location of

site), type of run (it will run through duct, trench,

buried etc.), number of tray, depth of trench,

distance between cables. Considering this condition

actual current currying capacity of cable reduce

than current currying capacity (which given to

cable Catalogue) this is called derating.

72:Why temperature rise is conducted in bus bars

and isolators?

Answer:Bus bars and isolators are rated for

continuous power flow, that means they carry

heavy currents which rises their temperature. so it is

necessary to test this devices for temperature rise.

73:When voltage increases then current also

increases then what is the need of

over voltage relay and over current relay? Can we

measure over voltage and over current by

measuring current only?

Answer:No.We can't sense the over voltage by just

measuring the current only because the current

increases not only for over voltages but also for

under voltage(As most of the loads are non-linear

in nature).So,the over voltage protection & over

current protection are completely different.

Over voltage relay meant for sensing over voltages

& protect the system from insulation break down

and firing. Over current relay meant for sensing any

internal short circuit, over load condition ,earth

fault thereby reducing the system failure & risk of

fire.So, for a better protection of the system.It

should have both over voltage & over current

relay.

74:If one lamp connects between two phases it will

glow or not?

Answer:If the voltage between the two phase is

equal to the lamp voltagethen the lamp will glow.

When the voltage difference is big it will damage

the lamp and when the difference is smaller the

lamp will glow depending on the type of lamp.

75:How do you select a cable size (Cu & Al) for a

particular load?

Answer:At first calculate the electrical current of

the load, after that derate the electrical current

considering derating factor(depending on site

condition and laying of cable) after choose the

cable size from cablecatalog considering derating

electrical current.After that measure the length of

cable required from supply point of load to load

poin. Calculate the voltage drop which will max 3%

(resistance and reactance of cable found from

cable catalog of selecting cable)

if voltage drop>3% then choose next higher size of

cable.

76:What are HRC fuses and where it is used?

Answer:HRC stand for "high rupturing capacity"

fuse and it is used in distribution system for

electrical transformers.

77:Which power plant has high load factor?

Answer:All base load power plants have a high

load factor. If we use high efficiency power plants

to supply the base load,we can reduce the cost of

generation.Hydel power plants have a higher

efficiency than thermal & nuclear power plants.

78:Mention the methods for starting an induction

motor?

Answer:The different methods of starting an

induction motorDOL:direct online starter,Star delta

starter,Auto transformer starter,Resistance

starter,Series reactor starter

79:What is the difference between earth resistance

and earth electrode resistance?

Answer:Only one of the terminals is evident in the

earth resistance. In order to find the second terminal

we should recourse to its definition: Earth



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Resistance is the resistance existing between the

electrically accessible part of a buried electrode and

another point of the earth, which is far away.

The resistance of the electrode has the following

components:

(A) the resistance of the metal and that of the

connection to it.

(B) the contact resistance of the surrounding earth

to the electrode.

80:What is use of lockout relay in ht voltage?

Answer:A lock-out relay is generally placed in line

before or after the e-stop switch so the power can

be shut off at one central location. This relay is

powered by the same electrical source as the control

power and is operated by a key lock switch. The

relay itself may have up to 24 contact points within

the unit itself. This allows the control power for

multiple machines to be locked out by the turn of a

single key switch.

81:What is the power factor of an alternator at no

load?

Answer:At no load Synchronous Impedance of the

alternator is responsible for creating

angle difference. So it should be zero lagging like

inductor.

82:Why most of analog o/p devices having o/p

range 4 to 20 mA and not 0 to 20 mA ?

Answer:4-20 mA is a standard range used to

indicate measured values for any process. The

reason that 4ma is chosen instead of 0 mA is for fail

safe operation .For example- a pressure instrument

gives output 4mA to indicate 0 psi, up to 20 mA to

indicate 100 psi, or full scale. Due to any problem

in instrument (i.e) broken wire, its output reduces to

0 mA. So if range is 0-20 mA then we can

differentiate whether it is due to broken wire or due

to 0 psi.

83:Two bulbs of 100w and 40w respectively

connected in series across a 230v supply which

bulb will glow bright and why?

Answer:Since two bulbs are in series they will get

equal amount of electrical current but as the supply

voltage is constant across the bulb(P=V^2/R).So

the resistance of 40W bulb is greater and voltage

across 40W is more (V=IR) so 40W bulb will glow

brighter.

84:What is meant by knee point voltage?

Answer:Knee point voltage is calculated for

electrical Current transformers and is very

important factor to choose a CT. It is the voltage at

which a CT gets saturated.(CT-current

transformer).

85:What is reverse power relay?

Answer:Reverse Power flow relay are used in

generating stations's protection. A generating

stations is supposed to fed power to the grid and in

case generating units are off,there is no generation

in the plant then plant may take power from grid.

To stop the flow of power from grid to generator

we use reverse power relay.

86:What will happen if DC supply is given on the

primary of a transformer?

Answer:Mainly transformer has high inductance

and low resistance.In case of DC supply there is no

inductance ,only resistance will act in

theelectrical circuit. So high electrical current will

flow through primary side of the transformer.So for

this reason coil and insulation will burn out.

87:What is the difference between isolators

and electrical circuit breakers? What is bus-bar?

Answer:Isolators are mainly for switching purpose

under normal conditions but they cannot operate in

fault conditions .Actually they used for isolating the

CBs for maintenance. Whereas CB

gets activated under fault conditions according to

the fault detected.Bus bar is nothing but a junction

where the power is getting distributed for

independent loads.

88:What are the advantage of free wheeling diode

in a Full Wave rectifier?

Answer:It reduces the harmonics and it also

reduces sparking and arching across the mechanical

switch so that it reduces the voltage spike seen in a

inductive load



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89:What are Motor Generator Sets and explain the

different ways the motor generator set can be used

?

Answer:Motor Generator Sets are a combination of

an electrical generator and an engine mounted

together to form a single piece of equipment. Motor

generator set is also referred to as a genset, or more

commonly, a generatorThe motor generator set can

used in the following different ways: 1.Alternating

current (AC) to direct current (DC), 2.DC to AC,

3.DC at one voltage to DC at another voltage, 4.AC

at one frequency to AC at another harmonically-

related frequency

90:What is power quality meter ?

Answer:Power Quality meters are common in

many industrial environment. Small units are now

available for home use as well. They give operators

the ability to monitor the both perturbations on the

power supply, as well as power used within a

building, or by a single machine or appliance. In

some situations, equipment function

and operation is monitored and controlled from a

remote location where communication is via

modem, or high-speed communication lines.So we

can understand the importance of power

measurement through power quality meters.

91:What is the different between digital phase

converter and ordinary phase converter?

Answer:Digital phase converter are a recent

development in phase converter technology that

utilizes proprietary software in a powerful

microprocessor to control solid state power

switching components. This microprocessor, called

a digital signal processor (DSP), monitors the phase

conversion process, continually adjusting the input

and output modules of the converter to maintain

perfectly balanced three-phase power under all load

conditions.

92:Explainthe operation of variablefrequency

transformer?

Answer:A variable frequency transformer is used

to transmit electricity between two asynchronous

alternating current domains. A variable frequency

transformer is a doubly-fed electric machine

resembling a vertical shaft hydroelectric generator

with a three-phase wound rotor, connected by slip

rings to one external ac power circuit. A direct-

current torque motor is mounted on the same shaft.

Changing the direction of torque applied to the

shaft changes the direction of power flow; with no

applied torque, the shaft rotates due to

the difference in frequency between the networks

connected to the rotor and

stator.The variablefrequency transformer behaves

as a continuously adjustable phase-shifting

transformer. It allows control of the power flow

between two networks .

93:What is the main use of rotary phase converter

?

Answer:Rotary phase converter will be converting

single phase power into true balanced 3 phase

power,so it is often called as single phase to three

phase converter .Often the advantages of 3 phase

motors, and other 3 phase equipment, make it

worthwhile to convert single phase to 3 phase so

that small and large consumers need not want to

pay for the extra cost of a 3 phase service but may

still wish to use 3 phase equipment.94Which type

of oil is used as a transformer oil?

Answers:Transformer oil, or insulating oil, is

usually a highly-refined mineral oil that is stable at

high temperatures and has excellent electrical

insulating properties. It is used in oil filled

transformers, some types of high voltage

capacitors, fluorescent lamp ballasts, and some

types of high voltage switches and circuit breakers.

Its functions are to insulate, suppress corona and

arcing, and to serve as a coolant.

Well into the 1970s, polychlorinated biphenyls

(PCB)s were often used as a dielectric fluid since

they are not flammable. They are toxic, and under

incomplete combustion, can form highly toxic

products such as furan. Starting in the early 1970s,

concerns about the toxicity of PCBs have led to

their banning in many countries.

Today, non-toxic, stable silicon-based or

fluoridated hydrocarbons are used, where the added

expense of a fire-resistant liquid offsets additional

building cost for a transformer vault. Combustion-

resistant vegetable oil-based dielectric coolants and

synthetic pentaerythritol tetra fatty acid (C7, C8)

esters are also becoming increasingly common as

alternatives to naphthenic mineral oil. Esters are

non-toxic to aquatic life, readily biodegradable, and

have a lower volatility and higher flash points than

mineral oil.



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95: If we give 2334 A, 540V on Primary side of

1.125 MVA step up transformer, then what will be

the Secondary Current, If Secondary Voltage=11

KV?

Answers :As we know the Voltage & current

relation for transformer-V1/V2 = I2/I1

We Know, VI= 540 V; V2=11KV or 11000 V; I1=

2334 Amps.

By putting these value on Relation-

540/11000= I2/2334

So,I2 = 114.5 Amps

96:what are the points to be consider for

MCB(miniature circuit breaker

selection?Answers:I(L)*1.25=I(MAX) maximum

current. Mcb specification are done on maximum

current flow in circuit.

97:what is the full form of KVAR?

Answers:We know there are three types of power

in Electricals as Active, apparent & reactive. So

KVAR is stand for ``Kilo Volt Amps with Reactive

component.

98:What is excitation?

Answer:Excitation is applying an external voltage

to DC shunt coil in DC motors.

99:In three pin plug 6 Amp. 220v AC rating. why

earth pin diameter is higher than ohter two pin?

what its purpose ?

Answer:Because Current flow in the conductor is

inversely proportional to the conductor diameter.

So if any short circuits occur in the system first

high currents bypassed in the Earthling terminal.(

R=Pl/a area of the conductor increases

resistance value decreases)

100:Different between megger and

contact resistance meter?

Answers:Megger used to measure cable resistance,

conductor continuity, phase identification where as

contact resistance meter used to measure low

resistance like relays ,contactors.

101:When we connect the capacitor bank in series ?

Answers:we connect capacitor bank in series to

improve the voltage profile at the load end in

transmission line there is considerable voltage drop

along the transmission line due toimpedance of the

line. so in order to bring the voltage at the load

terminals within its limits i.e (+ or - %6 )of the

rated terminal voltage the capacitor bank is used in

series

102:What is Diversity factor in electrical

installations?

Answers:Diversity factor is the ratio of the sum of

the individual maximum demands of the various

subdivisions of a system, or part of a system, to the

maximum demand of the whole system, or part of

the system, under consideration. Diversity factor is

usually more than one.

103:Why field rheostat is kept in minimum

position while armature rheostat at maximum

position?

Answers:In motors at the time of starting the

armature resistance is introduced to reduce the high

starting current and the field resistance is kept

minimum to have high starting torque.

104:Why humming sound occurred in h HT

transmission line?

Answers:This sound is coming due to ionization

(breakdown of air into charged particles) of air

around transmission conductor. This effect is called

as Corona effect, and it is considered as power loss.

105:what is rated speed?

Answers:At the time of motor taking normal

current (rated current)the speed of the motor is

called rated speed. It is a speed at which any system

take small current and give maximum efficiency.

106:What is different between grounding and

earthings?

Answers:Grounding means connecting the neutral

point of the load to the ground to carry the residual

current in case of unbalanced conditions through

the neutral to the ground whereas earthing is done



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in an electric equipment in order to protect he

equipment in occurence of fault in the system.

107:Why should be the frequency is 50 hz only

why not others like 45, 95 56 or anything , why

should we maintain the frequency constant if so

why it is only 50 ?

Answers:We can have the frequency at any

frequency you like, but than you must also make

your own motors, transformers or any other

equipment you want to use.We maintain the

frequency at 50hz or 60hz cos the world maintains

a standard at 50 /60hz and the equipments are are

made to operate at these frequency.

108:Why transformer rating in KVA instead of

KW?

Any device is rated depending on losses of the

machine. In a transformer the losses are iron losses

which depends on voltage and copper losses which

depends on current so there are no losses which

depend on power factor hence transformer is rated

in KVA.

There are 2 losses in transformer. One is copper

loss which depends on current and the other is iron

loss which depends on voltage. These two factors

are not affected by the power factor. This is why

transformers are rated in KVA and not KW. Single

phase KVA = Amps x Volts/1000. Single phase

KW = Amps x Volts x pf/1000. 3 phase KVA =

Amps x Volts x 1.73/1000. 3 phase KW = Amps x

Volts x 1.73 x pf/1000

yes there are two type of losses in transformer is

that ohmic losses and core losses . ohmic losses due

to transformer current and core losses due to

transformer voltage. these losses are due to

transformer voltage and current. and these losses

not depends on load ie kw that's the way its rating

in kva..

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